Radio-frequency identification (RFID) based toll collection systems typically use a single reader and associated antenna per lane of traffic. In such arrangements, an antenna is oriented such that its field of transmission and reception is aimed toward a lane of traffic, for example a road lane. The antenna associated with each lane of traffic is directed toward that lane and limited so that the field covered by that antenna does not overlap into neighboring lanes.
For example, in FIG. 1, a schematic elevation view of a four lane traffic pattern is illustrated. In this arrangement, a single antenna (illustrated as the triangular element) is associated with each lane, typically by being placed above and oriented downward toward the lane (typically oriented slightly back “upstream” toward oncoming traffic as well). Each antenna has a dedicated RFID reader that transmits RFID read requests and receives responses on an antenna port to which each antenna is respectively connected.
When the lanes are controlled to prevent vehicles passing between lanes of a multilane road (e.g., by including barriers between lanes), this arrangement can be effective. However, tolling is increasingly performed without barriers or other controls placed between lanes. In such situations, a number of problems occur. First, coverage of the RF field generated by each antenna is limited, so tags passing between readers will have reduced read rates. Second, if antenna fields are designed to overlap, those overlapping fields must be duty-cycled to prevent the antenna fields from both being active at the same time. This is because, if two readers attempt to communicate with an RFID tag at the same time, that RFID tag can become “confused” and fail to respond appropriately to either reader. This results in the tag not being read by either reader. It has been observed that, even if two adjacent readers are synchronized, efficiency of the readers decreases by more than fifty (50) percent.
Existing attempts to address this lack of efficiency use a single RFID reader having multiple antenna ports, with one antenna associated with each port, and one antenna per lane of traffic. The single RFID reader is then tasked with ensuring that no overlapping antennas are on at the same time, typically by turning on only one antenna at a time and cycling through the antennas. However, when used with two lanes of traffic, this arrangement causes efficiency to drop to less than fifty (50) percent, and for four lanes the efficiency of the antenna arrangement is less than twenty-five (25) percent (since only one antenna would be on, associated with a single lane, at any given time). Additionally, this single reader, one antenna per lane arrangement does not provide complete coverage across all lanes of traffic. Additional antennas added to each lane (e.g., two antennas directed to a common lane and activated by a single port of a reader) do not address the complete coverage issue because standing waves and resulting nulls are formed, in which an RFID tag would not respond.
For these and other reasons, improvements are desirable.